1. Field of the Invention
This invention relates to an apparatus for rapidly separating catalyst from a cracked hydrocarbon gas in a fluidized catalytic cracking (FCC) unit. The invention is also a process for withdrawing stripper gas from an FCC reactor vessel.
2. Related Apparatus and Methods in the Field
The fluid catalytic cracking (FCC) process comprises mixing hot regenerated catalyst with a hydrocarbon feedstock in a transfer line riser reactor under catalytic cracking reaction conditions. The feedstock is cracked to yield gasoline boiling range hydrocarbon as well as degradation products, such as coke which deposits on the catalyst causing a reduction in catalytic activity. Hydrocarbon vapor and coked catalyst are passed from the top of the riser reactor directly to a separator vessel wherein catalyst is separated from hydrocarbon. In the FCC art, the separator vessel is termed the reactor vessel. The separated catalyst is passed to a stripper wherein it is contacted with a stripping gas to remove volatile hydrocarbon. Stripped catalyst is then passed to a separate regeneration vessel wherein coke is removed from the catalyst by oxidation at a controlled rate. Catalyst, substantially freed of coke, is collected in a vertically oriented regenerated catalyst standpipe. The catalyst is passed from the standpipe to the riser reactor for cyclic reuse in the process.
A conventional feedstock comprises any of the hydrocarbon fractions known to yield a liquid fuel boiling range fraction. These feedstocks include light and heavy gas oils, diesel, atmospheric residuum, vacuum residuum, naphtha such as low grade naphtha, coker gasoline, visbreaker gasoline and like fractions from steam cracking.
Catalyst development has improved the fluid catalytic cracking (FCC) process. The fluid catalytic cracking process has been modified to take advantage of high activity catalysts, particularly crystalline zeolite cracking catalysts, to take advantage of the high activity, selectivity and feedstock sensitivity of these catalysts. These high activity catalysts has been used to improve the yield of more desirable products from feedstocks.
The hydrocarbon conversion catalyst employed in an FCC process is preferably a high activity crystalline zeolite catalyst of a fluidizable particle size. The catalyst is transferred in suspension or dispersion with a hydrocarbon feedstock, upwardly through one or more riser conversion zones which provide a hydrocarbon residence time in each conversion zone in the range of 0.5 to about 10 seconds, typically less than about 8 seconds. High temperature riser hydrocarbon conversions, occurring at temperatures of at least 900.degree. F. up to about 1450.degree. F., pressures of 5 psig to 45 psig and at 0.5 to 4 seconds hydrocarbon catalyst residence time in the riser are desirable. The vaporous hydrocarbon conversion product is rapidly separated from the catalyst.
Rapid separation of catalyst from hydrocarbon product is particularly desirable to constrain hydrocarbon conversion time to the residence time in the conversion zone. During the hydrocarbon conversion, coke accumulates on the catalyst particles and entrains hydrocarbon vapors. Entrained hydrocarbon contact with the catalyst continues after removal from the hydrocarbon conversion zone until the hydrocarbon is separated from the catalyst. The separation is typically by cyclone separating followed by stripping the catalyst with a stripping gas to remove volatizable hydrocarbon. Hydrocarbon conversion products and stripped hydrocarbon are combined and passed to a fractionation and vapor recovery system. This system comprises a fractionation tower, vapor coolers and wet gas compressor operated at a suction pressure of 0.5 to 10 psig. Stripped catalyst containing deactivating amounts of coke, is passed to a catalyst regeneration zone.
Cyclone separators are used to separate fluidized catalyst particles from cracked hydrocarbon. In a typical cyclone separator, a suspension of hydrocarbon vapor and entrained finely divided solid particulate catalyst is introduced tangentially into the separator barrel. In the barrel a spiral, centrifugal motion causes the solid particles to be thrown to the wall of the cyclone separator where they flow downward under the force of gravity to a catalyst bed. Separated vapor is removed through an axial vapor withdrawal conduit extending below the tangential inlet conduit upwardly through the top of the cyclone separator. A vapor recovery system, in fluid communication with the vapor withdrawal conduit, is maintained at reduced pressure to assist the withdrawal of vapor from the cyclone separator.
An object of the present invention is to provide an apparatus particularly suited for rapidly separating the catalyst-hydrocarbon suspension. Another object is to establish a stable pressure gradient between the cyclone barrel and the reactor vessel to facilitate removing stripper gas from the reactor vessel. Another object of this invention is to provide a cyclone separator apparatus which withstands thermal expansions.
Perry's Chemical Engineers, Handbook, 4th ed., pp. 20-68 to 20-71 describes general design parameters for cyclone separators used for removing solid particles from vapors.
Kirk-Othmer Encyclopedia, 3rd ed., Vol. 1, pp. 667 to 672 describes general design parameters for cyclone separators used for separating solid particles from gases.
U.S. Pat. Nos. 4,623,446 and 4,737,346 to J. H. Haddad et al. teach a closed coupled cyclone separator system in the reactor vessel of a fluid catalytic cracking apparatus. Means is provided for blending stripping gas with cracked hydrocarbon as it flows to a directly coupled riser cyclone separator. As shown in FIG. 7 and 8, the riser reactor conduit is modified to comprise an overlapping downstream portion 118 to provide an annulus between the upstream portion 117 and the downstream portion 118. The annulus is covered by a flat metal ring having orifices 125 in open communication with the reactor vessel, enabling stripping gas to pass into the downstream conduit 118. A riser cyclone dipleg is sized, as seen in FIG. 5, to admit at least a portion of stripping gas from the stripping zone to pass countercurrent to catalyst passing downwardly through the dipleg.
U.S. Pat. No. 4,502,947 to Haddad et al. discloses a closed cyclone fluid catalytic cracking catalyst separation method and apparatus. In the closed cyclone, hydrocarbon product and catalyst are passed directly into a cyclone separator from a riser without passing into the atmosphere of the reactor vessel. Avoiding the atmosphere of the reactor vessel reduces both excess catalytic cracking and high temperature thermal cracking.